NOVEL COMPUTATIONAL MODEL FOR THE INTERACTION OF DOPAMINE WITH THE D2-RECEPTOR

With the use of molecular modelling and computational chemistry, two m odels were obtained showing the binding of dopamine to part of the D2 receptor. These models consist of a molecular complex containing dopam ine, the transmembrane domains (TM) 3, 4 and 5, a small part of the ex tracellular loop just before TM3 and a larger part of the loop connect ing TM4 and TM5. The models are further characterized by the presence of two disulphide bonds: one between TM3 and TM4 and a second linking the Cys residue before TM3 with the Cys residue in the loop between TM 4 and TM5. In the first model, the beta-adrenergic receptor is used as a reference with the catechol moieties of dopamine interacting with t wo Ser residues in TM5, and the Asp residue in TM3 interacting with th e protonated nitrogen of dopamine. In the second model, dopamine is po sitioned between the TM without interaction assumptions. In both model s the TM positions are close to the arrangement of the alpha-helix ord ering as found in bacteriorhodopsin. Energy minimization was accomplis hed using these two starting structures. The two minimized complexes s how different molecular interactions that make good chemical sense and are in agreement with reported X-ray data of different proteins. The disulphide bond between TM3 and 4 is apparently unique to the D2 and D 3 receptors. The first minimized complex revealed that not Ser194, as in the beta-receptor, but Ser193 interacted with the meta-hydroxyl of dopamine, although a proper hydrogen bond with Ser194 was initially pr esent in the starting structure. The para-hydroxyl group interacted wi th Ser197. The role of Ser193 and not Ser194 in the binding of dopamin e is in agreement with recently reported site-directed mutagenesis stu dies. The second minimized complex was the more stable. In this comple x the hydroxyl groups of dopamine interacted with the extracellular di sulphide bond formed between Cys107 and Cys182, Ser 193 interacted wit h the phenyl ring and Ser194 again played no role. These modelling dat a could explain our recent biochemical findings on the effect of sulph ydryl reagents on the binding of ligands to the D2 receptor. From the obtained results it is also concluded that the residues Cys107,118,168 ,182, Leu174, Phe110,172,198,202 and Asp114 constitute good targets fo r experiments using site-directed mutagenesis.